DSD+ & DMR

[RonnieUSA]

I have noticed that when using my tapped scanner to decode DMR, it shows +DMR, but if I use my dongle it shows -DMR, both are clean decodes with no errors, what is the reason behind this?

 

[slicerwizard]

Your devices are presenting raw audio with differing polarities.

I just took a look at SDR# (don't normally use it - far too much CPU usage) and I see that as a transmitter's frequency goes up, the demodulated audio goes more negative. Looks like DSD+ is expecting the opposite (positive RF modulation, positive audio shift / negative RF modulation, negative audio shift)

 

[RonnieUSA]

Well I just tried it using HDSDR, and it's showing +DMR.
Could this be a SDSharp flaw?
The second screenshot is after going back to SDRSharp.

 

[EricCottrell]

Hello,

On my R2500 I get both + and - depending on the frequency band. There is nothing wrong, it is just how the receiver is designed.

C4FM modulation data values are defined as four frequency shifts, two positive shifts and two negative shifts. The frequency shifts result in positive and negative voltage levels out of the discriminator tap of the receiver.

The various processing stages of a receiver can invert the signal. This results in a different data value than intended. The decoding software can invert the data again to correct the data value.

Regular analog audio is not polarity sensitive, so signal inversion by the receiver does not matter.

73 Eric

 

[RonnieUSA]

Hello,

On my R2500 I get both + and - depending on the frequency band. There is nothing wrong, it is just how the receiver is designed.

C4FM modulation data values are defined as four frequency shifts, two positive shifts and two negative shifts. The frequency shifts result in positive and negative voltage levels out of the discriminator tap of the receiver.

The various processing stages of a receiver can invert the signal. This results in a different data value than intended. The decoding software can invert the data again to correct the data value.

Regular analog audio is not polarity sensitive, so signal inversion by the receiver does not matter.

73 Eric

Thank You for the explanation.

 

[slicerwizard]

The various processing stages of a receiver can invert the signal. This results in a different data value than intended. The decoding software can invert the data again to correct the data value.

IMO, SDR software should not pick a random polarity. It would be nice if the developers could agree on one or the other. I vote for positive deviation = positive audio shift.